U.S. Pat. No. 5,871,823 [Anders, Hoecker, Klee, and Lorenz] [1] reports using UV light in the wavelength range of 125-310 nm to activate polymer surfaces in the presence of oxygen with a partial pressure of 2×10−5 to 2×10−2 bar. The activated surface is subsequently grafted. However, this patent is limited to the use of surface hydroperoxides obtained from UV activation to initialize grafting.
U.S. Pat. No. 5,629,084 (Moya, Wilson) [4] discloses a composite porous membrane formed from a porous polymeric substrate and a second polymer which has been cross-linked by heat and UV. The modification of the second polymer is over the entire surface, which is attained by placing a membrane in contact with a second polymer solution and initiator and exposing everything to UV or mild heat in order to crosslink a second polymer on the substrate surface. This scheme can be categorized as a “grafting to” technique where the adsorption of a second polymer to the fiber surface is the critical step.
UV-initialized grafting is generally performed by exposing the substrate to UV light in monomer solutions. It can take place in the range 100-450 nm for a variety of molecules. U.S. Pat. No. 5,871,823 [Anders, Hoecker, Klee, and Lorenz] [1] reported using a preferred UV wavelength in the range 290-320 nm. PCT/WO/02/28947 A1 [Belfort, Crivello and Pieracci] [5] reported using UV wavelengths in the range 280-300 nm. These inventions do not refer to the use of a photosensitizer in the grafting process.
In addition, U.S. Pat. No. 5,468,390 [Crivello, Belfort, Yamagishi] [6] discloses a process to modify polysulfone porous membranes without photosensitizers. As a result, only the outer surface of the membranes described in this reference was modified through the treatment. The polysulfone membranes cannot be rewetted after drying.
U.S. Pat. No. 5,883,150 [Charkaudian] [7] reports that implanting a photosensitizer into the backbone of the polysulfone membrane results in better wetting properties. Nonetheless, it is difficult for most of these implanted photosensitizers to survive the high temperature conditions that are generally used for polymer processing. For example, fiber or nonwoven production with melt-blowing processes requires temperatures above 120° C.
In summary, while surface modification methods such as those described above may generate some coatings on the fiber surface of fiber nonwoven webs or mats, a conformal coating cannot be assured by these methods because they do not provide the necessary means either to overcome possible differences between the surface energies of the substrate and second polymers, or to generate a surface with a high density initiator.
It is, therefore, desired to have a surface modification method which can warrant conformal coating for a wide range of polymer fibers. It is also desired that this method be robust and easy to scale-up. The present invention seeks to meet these and related needs.